The structure of a dynamo electric machine plays an important role in the limits to the mechanical capacity of the machine. A further limiting factor is power electronics capabilities which provide the electrical drive to the motor.
Electric motors are often operated with electronic variable speed drive systems, which adjust the voltage and current being supplied to the motor in order to operate the motor at a desired speed under a given torque load, or to control the torque load to a fixed value, or to otherwise control the mechanical output of the motor. A particularly useful type of variable speed drive is the polyphase alternating current inverter, which may be used to operate polyphase AC motors.
In many applications, a fixed amount of input electrical power is available, and this is to be converted to mechanical output power as needed. It is desirable that the full available electrical power be available for conversion to mechanical power over a wide speed range.
At low speeds, it is often desired to increase the motor output torque, as mechanical output power is the product of torque and speed, allowing the full use of available electrical power. Torque is related to the slot current supplied to the motor windings, the slot current being the total current of all of the conductors wound in parallel through the same slot. The greater the current in each of the windings, the higher will be the slot current. The current in each of the windings is however limited to inverter maximum current ratings.
A further way to increase the slot current is at the design stage, to wind the motor with more stator winding turns. A motor wound with many turns will, for the same input current from the power electronics, produce a greater slot current, and thus more torque, than a motor with fewer series turns. However, this increase comes at a cost; the increased number of series turns raises the voltage of the winding, and increases the voltage required to feed the current into the winding.
At low speeds, this increased voltage requirement is not a problem, as the voltage of a winding is also related to speed, and is lower at reduced speed. However, this increased voltage requirement will decrease the maximum speed at which the inverter can supply sufficient voltage to the motor. Voltage too is limited to inverter ratings.
The maximum voltage that an inverter can produce is set by the DC link voltage; this sets the peak amplitude of the output waveform. If a pure sine wave is synthesized, then the RMS voltage of this sine wave is directly related to this peak value (for sine waves, RMS voltage peak=voltage/v2). However, adding the third harmonic to the waveform can change the relationship between RMS voltage and the peak amplitude value. This technique is well known in the art of three-phase inverter driven machines.
Known to the art of three-phase machines is to selectively add third harmonic to the inverter output waveform. By adding suitable third harmonic to a waveform, the peak amplitude of the waveform is reduced, all the while maintaining the exact same fundamental amplitude. The third harmonic voltage does not cause third harmonic currents to flow into the three-phase machine; so the machine itself sees unchanged fundamental. By then increasing the total amplitude of the waveform, the amplitude of the fundamental component of the output waveform can actually be increased above the peak output voltage of the inverter.
The present invention increases the torque or speed capabilities of a high phase order motor drive system by increasing the utilization of the active switching elements while remaining within the voltage and current limitations of same. A range of harmonics is added to the inverter output waveform to increase the output voltage capacity of the inverter. Harmonics may also be used to adjust the current waveform feeding the motor, thereby increasing the current output capability of the inverter. In a preferred embodiment, the motor is wound with an increased number of series turns to translate the additional available voltage into additional available slot current. In an alternative embodiment, a inverter voltage waveform is synthesized, producing an increased current output.